Pogo and Other Problems

The pogo bounce had been observed (although to a much smaller degree) on
Apollo 4, so its appearance during Apollo 6
did not come as a complete surprise. Also, five years earlier, in 1963,
pogo had threatened to end the Gemini program when the Titan II suffered
this phenomenon on launch after launch. Its apparent cause was a partial
vacuum created in the fuel and oxidizer suction lines by the pumping
rocket engines. This condition produced a hydraulic resonance - more
simply, the engine skipped when the bubbles caused by the partial vacuum
reached the firing chamber. Sheldon Rubin of the Aerospace Corporation
had finally suggested installing fuel accumulators and oxidizer
standpipes, to ensure a steady flow of propellants through the lines.
This had solved the Gemini launch vehicle problems, and NASA had this
background experience to draw on when the Saturn V began having pogo
troubles.*39

Pogo on Apollo 4 had been measured at one-tenth g, much
less than the one-fourth g set as the upper limit in Gemini. The lower
oscillation was probably the result of carrying just "a hunk of
junk," to simulate lunar module weight, on the earlier flight. But
a test article flown on Apollo 6 had the shape and weight
of a real lander in the adapter. This change in mass distribution
coupled back into the fuel system problem and increased the pogo
oscillations. The mission analysts later discovered that two of the
Saturn engines had been inadvertently tuned to the same frequency,
probably aggravating the problem. (Engines in the Saturn V cluster were
to be tuned to different frequencies to prevent any two or more of them
from pulling the booster off balance and changing its trajectory during
powered flight.)

The rocketeers at Huntsville first wanted to know from Houston whether a
crew could have withstood the vibration levels on Apollo 6.
If so, the next Saturn V flight could be manned, even without a pogo
cure. Low informed Saturn V Program Manager Arthur Rudolph that these
levels could not be tolerated. Marshall also asked whether the emergency
detection system could be used to abort the mission automatically if
such high vibrations again occurred. During Apollo 6, the
system had cast one vote for ending the mission. Had it cast a second
vote, abort would have been mandatory. Low and chief astronaut Donald
Slayton did not want to use the system in an automatic pogo abort mode.
Low met with George H. Hage, Phillips' deputy, and they decided on the
immediate development of a "pogo abort sensor," a
self-contained unit that would monitor and display spacecraft
oscillations. From what the sensor told him, a spacecraft commander
could decide whether to continue or stop the mission.40

Marshall Space Flight Center pulled an S-IC stage out of Michoud
Assembly Facility, brought it to Huntsville, and erected it in a test
stand. By May, Huntsville, Houston, and Washington Apollo officials were
ready to attack the pogo problem. Hage agreed to head the activity until
Eberhard Rees could finish his task on the command module at Downey and
take over. At one time during the pogo studies, Lee B. James (who had
replaced Rudolph as the Huntsville Saturn V manager) said, 1,000
engineers from government and industry were working on the problem.41

Out on the West Coast, at the rocket engine test site at Edwards Air
Force Base, Rocketdyne started testing its F-1 engine in late May. In
the first six tests, helium was injected into the liquid-oxygen feed
lines in an attempt to interrupt the resonating frequencies that had
caused the unacceptable vibration levels. In four of the six tests, the
cure was worse than the disease, producing even more pronounced
oscillations. The Saturn V people at Marshall also tried helium
injection, but their results were decidedly different. No oscillations
whatsover were observed. Tests using the S-IC stage's prevalves as
helium accumulators were then conducted at both Edwards and Marshall.
The prevalves were in the liquid-oxygen ducts just above the firing
chambers of the five engines and were used to hold up the flow of oxygen
in the fuel lines until late in the countdown, when the fluid was
admitted to the main liquid-oxygen valves in preparation for engine ignition.
The prevalves were modified to allow the injection of helium into the cavity
about 10 minutes before liftoff; the helium would then serve as a shock
absorber against any liquid-oxygen pressure surges.

What had happened to the S-II and S-IVB stages, with two of the five J-2
engines shutting down in one case and the single J-2 engine refusing to
start in the other, was more of a mystery than pogo. During tests at
Arnold Engineering Development Center, at Tullahoma, Tennessee,
engineers discovered that frost forming on propellant lines when the
engines were fired at ground temperatures served as an extra protection
against lines burning through. But frosting did not take place in the
vacuum of space; the lines could have failed because of this. Also, in
the line leading to each of the engines was an augmented spark igniter.
Next to the igniter was a bellows. During ground tests, liquid air,
sprayed over the exterior to cool it, damped out any vibrations. Vacuum
testing revealed that the bellows vibrated furiously and failed
immediately after peak-fuel-flow rates began. These lines were
strengthened and modified to eliminate the bellows.42

Another item noticed by the flight control monitors during the boosted
flight of Apollo 6 (and later confirmed by photographs) was
that a panel section of the adapter that housed the lander had fallen
away just after the Saturn V started bouncing. The controllers had been
amazed that the structural integrity was sufficient to carry the payload
into orbit. James Chamberlin in Houston discovered that thermal pressure
(and therefore moisture) had built up in the honeycomb panels during
launch; with no venting to allow the extra pressure to escape, the panel
had blown out. A layer of cork was applied to the exterior of the
adapter to keep it cooler and to absorb the moisture, and holes were
drilled in the adapter panels to relieve the internal pressure if heat
did build up inside on future launches.43

Although Marshall was responsible for stability and dynamic structural
integrity throughout the boost phase, the Manned Spacecraft Center could
not afford to sit on the sidelines and watch while its sister center
wrestled with these problems. Houston had to get an Apollo payload stack
together for structural testing. On 16 May 1968, Low and James decided
to use a "short stack" (the S-IC stage would be left out at
this time but could be incorporated later).** Astronaut Charles Duke was sent to
Huntsville to keep information flowing between the centers, and Rolf
Lanzkron was assigned by Low to manage the spacecraft dynamic integrity
testing, which was satisfactorily completed on 27 August with no major
hardware changes found necessary.44

* The Gemini launch vehicle engines
were hypergolic, that is, its oxidizer and fuel burned on contact to
produce thrust. Since the Saturn first stage (S-IC) engines were
cryogenic, the propellant and oxidizer needed an igniter to produce
burning - and no one expected a similar pogo problem with the larger
booster.

** The stack comprised an S-IVB
forward skirt, launch vehicle instrument unit, spacecraft-lunar module
adapter, LM-2, a service module, a Block I command module, and the
launch escape system from boilerplate 30.